The stress concentration at the edge of holes shortens fatigue lives of structures and machinery. To avoid premature fatigue failures, the design stress can be decreased to an appropriate low value. However, such a decreased design stress value will result in an increase in weights of the structures and machinery. Metal fatigue is a peculiar design concern in the aircraft industry because of the stringent light weight design requirement and the abundance of holes in the airframe structures. The fatigue life of holes can be enhanced by hole coldworking which creates a beneficial compressive residual stress at hole edge. Several prior art methods are available to enhance fatigue lives of holes, for example, split sleeve method described in U.S. Pat. No. 3,566,662, seamless sleeve method described in U.S. Pat. No. 4,164,807, stress coining method described in U.S. Pat. No. 3,895,922, and split mandrel method described in U.S. Pat. No. 4,665,732. The split sleeve method is the common method currently used in the aircraft industry. The disadvantages of the prior art of split sleeve and split mandrel methods are:
i) The split in the sleeve or the split in the mandrel creates a ridge and shear discontinuity in the wall of coldworked hole. The ridge and shear discontinuity reduce the effectiveness of coldworking to enhance fatigue life. PA1 ii) A final reaming step is often required to clean up the ridge and shear discontinuity, and to size the coldworked holes. The final reaming step requires extra labor and prolongs process time; therefore, it incurs significant manufacturing cost. PA1 iii) The final reaming step removes material from a coldworked hole, thereby reduces the beneficial compressive residual stress at the hole edge, and thereby reduces the effectiveness of coldworking. PA1 i). Eliminates the aforementioned disadvantages of the split sleeve, split mandrel, and seamless sleeve methods. PA1 ii). Reduces the time to perform coldworking because there is no need to do final reaming of the hole after coldworking, and thereby reduces manufacturing cost. PA1 iii). Preserves higher beneficial compressive residual stress at hole edge because the invention eliminates the final reaming step which relieves portion of the compressive stress. This will result in better fatigue life enhancement. PA1 iv) Achieves higher compressive residual stress at hole edge with lower interference level of coldworking. This advantage will make it easier to coldwork those holes in the short transverse direction of the high strength aluminum thick plates wherein ductility is low and is susceptible to stress corrosion.
The prior art of split mandrel method does not use a sleeve to protect the hole in a workpiece; this would save time and cost. However, it has the propensity to create galling on the hole wall because there is no medium between the hole and split mandrel to protect the hole wall. It also suffers the aforementioned disadvantages because of the split in the mandrel.
The prior art of seamless sleeve method does not create ridge and shear discontinuity in the hole wall and does not require a final reaming step; it should result in better fatigue life improvement than the prior art of split sleeve and split mandrel methods. However, the disadvantage of this prior art method is that the plastically deformed seamless sleeve become stuck with the hole wall and is under the compressive stress exerted by the hole wall. Thus, it is difficult to remove the sleeve out of the hole after coldworking. To leave the seamless sleeve in the coldworked hole for isolating the fretting phenomenon has not been a desired practice because the sleeves made of steels increase the weight of airframe structures. Another disadvantage of this prior art method is that it requires the mandrel to be taken off the mandrel pulling means for assembling the seamless sleeve onto the small diameter portion of the mandrel. The prior art of various stress coining methods, for example, radius coining, pad coining, and ring pad coining, have limited application because of the relative complexity of the methods; they are primarily used for non-circular holes and very large circular openings where the prior art of split sleeve and split mandrel methods are not applicable. All of the aforementioned disadvantages of the prior art of coldworking methods can be eliminated by the present invention which utilizes a tubular seamless sleeve made of shape memory alloys.